Optical devices are sensitive to changes in temperature. For example, the output wavelength, the optical power output and the current threshold of a semiconductor laser are sensitive to temperature. Similarly, the effective refractive indices of optical modes in a semiconductor or glass waveguide also change with temperature. Typically, optical devices and systems are designed to account for such thermally-induced shifts, at least in those device/system parameters that are the most important for proper performance of the device/system.
Dense wavelength divisional multiplexed (DWDM) systems boost information-carrying capacity by employing multiple sources of differing wavelength to broadcast multiple signals over a single fiber. As the density of the fiber channels increases, the requirements of wavelength stability of each channel becomes more stringent. Currently, DWDM systems use channels spaced apart by 50 or 100 GHz. To achieve wavelength stability over a 25-year system lifetime, optical sources need wavelength stabilization. One way to provide such wavelength stabilization is by controlling the thermo-optic behavior of the system components over an operating temperature range ΔT.
In a DWDM system, the channel wavelengths are packed together at minimal spacing. Therefore, any significant drift in the output wavelength of the lasers (used as carrier signal sources) poses serious problems. One source of wavelength drift is aging of the lasers. Wavelength changes due to aging can be corrected by changing the temperature of the laser by a thermoelectric cooler/heater (TEC). Although the wavelength of each laser can be controlled by an etalon used as a frequency discriminator, the optical path length of the etalon is also temperature sensitive. Thus, changing the temperature of the laser might also change the temperature of the etalon if the two devices are located in the same package. Nevertheless, other phenomena can also affect the temperature of the etalon and hence its optical path length. For example, (1) aging of the temperature control circuit used to maintain the laser at a predetermined temperature and at a predetermined output wavelength, and/or (2) the temperature gradient produced across the etalon because the TEC is typically located on the base of the etalon could also affect the temperature of the etalon.
Accordingly, there is a need for controlling the thermo-optic behavior of an optical path within optical devices and over a temperature range. There is also a need for an optical etalon used as wavelength discriminator that is essentially temperature insensitive over the temperature operating range of the corresponding optical system.